Innate immunity is the first line of defense against invading microorganisms. The Toll-like receptor (TLR) family, a group of recently defined innate immune receptors, plays a central role in detecting conserved microbial components. The core TLR signaling pathway utilizes myeloid differentiation factor 88 (MyD88) as the primary adaptor; while others utilize alternative adaptors such as TIRAP/Mal, TRIF/TICAM-1 and TIRP/TRAM/TICAM-2. Upon viral infection, stimulation of TLR signaling leads to activation of NF- kB and release of cytokines, including both anti-viral factors and proinflammatory cytokines. The effects of TLR signaling could be favorable and/or potentially detrimental to the host immunity. We have recently demonstrated that West Nile (WN) virus infection in mouse leads to a TLR3-dependent inflammatory response that is involved in both brain penetration of the virus and neuronal injury thereby contributing to lethal encephalitis. Furthermore, we found in the preliminary studies that MyD88 deficient (MyD88-/-) mice were more susceptible to WN virus infection than wild-type mice, suggesting MyD88-mediated signaling is involved in protective immunity against WN virus. TLR3 responds in a MyD88 independent fashion; TLRs 7 and 8 are currently identified MyD88-dependent TLRs that are involved in viral RNA recognition. Based on these data, we hypothesize that MyD88-dependent TLR7/8 signaling is important for host protective immunity against WN virus infection. In Specific aim 1, we will further examine WN virus infection in both MyD88 deficient and TLRs 7 and 8 deficient mice. Specific aim 2 is to dissect the effector mechanisms that underlie MyD88-mediated innate immunity in control of WN virus infection. WN virus has now induced the largest outbreaks of viral encephalitis in North America and has become a public health concern. Results from this study will not only enhance our understanding of the viral pathogenesis; they will also lead to the development of new strategies to prevent and treat WN encephalitis. [unreadable] [unreadable] [unreadable]